Problem: An ice cream truck started the day with $45$ ice cream sandwiches. During a stop at a busy park they sold $s$ ice cream sandwiches. After the stop, the truck had $21$ ice cream sandwiches remaining. Write an equation to describe this situation. How many ice cream sandwiches did the truck sell at the park?
Answer: The truck started with a total of ${45}$ ice cream sandwiches. They sold an unknown number of sandwiches at the park, which we're calling ${s}$. The truck had ${21}$ ice cream sandwiches remaining after the stop at the park. We can represent the number of ice cream sandwiches the truck has remaining as a difference: ${45} - {s}$ We know the truck has ${21}$ ice cream sandwiches remaining. We can set these two expressions equal to describe this situation with an equation: ${45} -{s} = {21} $ Other ways to represent the situation with an equation include: ${45} - {21} = {s}$ or ${21} + s = {45} $. Now we can solve for ${s}$. Add ${s}$ to both sides of the equation then subtract ${21}$ from both sides: $\begin{aligned} {45} -{s} +{s} &= {21}+{s} \\ {45} &= {21}+{s} \\ {45} -{21} &= {21}-{21}+{s} \\ {24} &= s \end{aligned}$ The following equation matches this situation: $45-s=21$ The ice cream truck sold $24$ ice cream sandwiches at the park.